(1) Field of the Invention
This invention concerns methods for the manufacture of brazed aluminum heat exchangers. In particular, the invention is directed to methods of manufacturing brazed aluminum heat exchangers by affixing brazed heat exchanger core modules together to form complete heat exchanger devices.
(2) Description of the Related Art
A common heat exchanger used in cryogenic, refinery and chemical applications is the plate-fin brazed aluminum heat exchanger. Typical brazed aluminum heat exchangers are fabricated by disposing corrugated aluminum sheets between aluminum parting sheets or walls to form a plurality of fluid passages. The aluminum parting sheets are typically 1 mm thick for lower operating pressures, and as thick as 4 mm for exchangers used in high pressure service. The sheets are either clad with an aluminum brazing layer or a layer of brazing foil is inserted between the surfaces to be bonded. When heated to a predetermined temperature for a predetermined period of time, the brazing foil or cladding melts and forms a metallurgical bond with the adjacent sheets. The resulting assembly contains numerous passages consisting of alternate layers of closely spaced fins. A typical arrangement of alternate layers of passages each containing fins with a density of 6 to 10 fins/cm (15 to 25 fins/inch), and a fin height of 0.5 to 1 cm (0.2 to 0.4 inch). In a common application of such heat exchanger cores, a first series of passages carry vapor for condensing, while a second series of passages, interleaved in an alternate fashion with the first series of passages, carry a liquid for boiling. Typical brazed aluminum heat exchangers must be able to withstand 2068 to 2758 kPa (300 to 400 psia). Brazed aluminum heat exchangers can be designed for operating pressures in excess of 1000 psia, but exchangers of most interest in this invention typically operate at pressures less than 300 psia.
FIG. 1 shows a diagrammatic cross-sectional representation of a prior art brazed aluminum heat exchanger core in a general form. The brazed aluminum heat exchanger core 20 includes a cap sheet 22 on a top or first end 24, and another cap sheet 26 on a bottom or second end 28 of the heat exchanger. The core 20 includes a plurality of finned passages 30. The finned passages 30 include boiling passages 32 and condensing passages 34 that are arranged in the core 20 in an alternate or interleaved fashion. Side bars 36 bracket the core 20. It will be understood that the core depicted is only a part of a heat exchanger, and that after the core is assembled and subjected to heating and thus, brazed into a heat exchanger core, hand welding is typically performed to complete manufacture of the heat exchanger, including the attachment of headers and manifolds as is known in the art.
Many conventional brazed aluminum heat exchangers typically contain fins in all passages and are brazed as a single unit in large, specially designed vacuum brazing furnaces. Only a handful of companies worldwide have the capability to manufacture very large heat exchanger cores and fixtured assemblies, in a size which is, for example, 4′×4′×24′ long. Due to the passages all containing fins, such as for example, plain fins, plain-perforated fins, serrated fins, herringbone or wavy fins, this design is best assembled and brazed in one step. Furthermore, the vacuum furnaces that can accommodate and effect the brazing of these size heat exchanger assemblies are very costly, and indeed, since a great deal of capital has been invested in vacuum brazing assemblies of this size, these companies prefer to braze heat exchangers as single cores.
Because of the large size of conventional heat exchangers, a furnace brazing cycle may be 24 hours or more. The large size of conventional heat exchanger cores, coupled with the fact that all of the passages of conventional cores are typically finned passages, leads to a scenario where manufacturers find it advantageous to assemble the cores into a single unit, and then braze the cores in a large vacuum oven in one step. Brazing of single core units is thought to reduce the number of steps, or amount of hand labor required in welding, which is attractive generally when automated processes are available.
As suggested above, one of the disadvantages of the single step brazing process is the requirement of using one of the few facilities where large vacuum brazing furnaces are available. Another disadvantage is the time it takes to perform the brazing operation, because of the size and configuration of the large core units. Because of the amount of time required to perform the brazing step, there exists the potential of uneven heating of parts of the heat exchanger core and the potential of resultant damage, such as distortion or bowing of core elements and over-brazing or under-brazing of core joints or elements.
Some heat exchangers include surfaces comprising enhanced boiling layers (EBLs). EBLs are well known in the art. Enhanced boiling layers were first proposed for heat exchangers in U.S. Pat. No. 3,384,154. This patent discloses mixing metal powder in a plastic binder in solvent and applying the slurry to a base metal surface. The coated metal is subjected to a reducing atmosphere and heated to a temperature for sufficient time so that the metal particles sinter (bond) together and to the base metal surface. U.S. Pat. No. 3,457,990 discloses an enhanced boiling surface with reentrant grooves mechanically or chemically formed therein.
Other methods of applying EBLs have been disclosed. GB 2 034 355 discloses applying an organic foam layer to a metal heat transfer member and plating the foam with metal such as copper first by electroless plating, then by electro-deposition. U.S. Pat. No. 4,258,783 discloses mechanically forming indentations in a heat transfer surface and then electrodepositing metal on the pitted surface. GB 2 062 207 discloses applying metal particles to a metal base by powder flame spraying. EP 303 493 discloses spraying a mixture of metal and plastic material onto a base metal by flame or plasma spraying. U.S. Pat. Nos. 4,767,497 and 4,846,267 disclose the heat-treating of an aluminum alloy plate to produce a precipitate followed by chemically etching away the precipitate to leave a pitted surface. EP 112 782 discloses applying a mixture of brazing alloy and spherical particles to a metallic wall and heating the coated wall to melt the brazing material.
Patents proposing replacing fins with an enhanced boiling layer in the boiling passages of a brazed heat exchanger include U.S. Pat. Nos. 5,868,199; 4,715,431 and 4,715,433. These patents propose to stack aluminum sheets each with an EBL applied on one side to define boiling channels and with fins on the other side of the aluminum sheets to define condensing channels. Layers of brazing material are disposed between bonding surfaces in the stack, and the stack is subjected to heating over a period of time to obtain a brazed heat exchange core. Such brazed aluminum heat exchangers described in these patents have not been commercialized because EBLs are typically brazed at 565 to 593 degrees C. (1050 to 1100 degrees F.) while the subsequent brazing of the metal components together occur at around 593 to 621 degree C. (1100 to 1150 degrees F.). Maintaining the integrity and effectiveness of the EBL, particularly the porous structure provided by the mutually bonded metal particles, during the second hotter heat treatment to effect brazing has been difficult. This difficulty accounts for the lack of commercially available brazed heat exchangers with EBL in the boiling passages. An additional concern for EBLs with the porous structure is the volume of gas products formed during the brazing of large heat exchangers, wherein there can be a problematic volume of gas produced during heating, which is directly related to the size of the fabricated units.
In vacuum brazing operations, such as those described above, the length of time of the heat cycle can contribute to the damaging of the enhanced boiling layer and thus, ultimately of the integrity and efficiency of the operation of the heat exchanger. One solution is proposed in U.S. Patent Publication No. 2008/0041573, which not only describes an effective EBL, but also describes how heat exchanger cores are provided with EBLs and how these heat exchanger cores and heat exchangers are manufactured.
There is a need for a method of manufacturing brazed aluminum heat exchangers in such a fashion so as to provide increased manufacturing flexibility while retaining or increasing the efficiency and integrity of brazed aluminum heat exchangers. The invention satisfies the need.